So close. They’re almost interchangeable. But they’re not quite.
Yeah is a slang/casual form of yes. Yea is not, though it is often used as such. Yea originally referred to an affirmative vote: yea or nay. These days, though, it’s used as a cheer. “Yea! We won!”
The proper word for that is yay. Nowadays, spellcheck will flag that spelling, so waddaya gonna do? The only one that ever bothers me is when people used yea to say yes.